Intestinal mantle cell lymphoma characteristically produces multiple polyps, a finding reported as multiple lymphomatous polyposis. The early stages of intestinal mantle cell lymphoma before polyp formation and the pattern of initial lymph node invasion, however, have not been described. We recently encountered two cases of intestinal mantle cell lymphoma in their early development found incidentally associated with advanced colonic adenocarcinoma. We present herein the clinical, histopathological, immunohistochemical, and molecular genetic features of these two cases. In one case, a single polypoid mass was found with invasion limited to mucosa and submucosa of the terminal ileum and without lymph node compromise. In the second case, there were multiple mucosal aggregates of neoplastic cells without formation of polyps. Regional lymph nodes in the latter case showed either partial or complete involvement by lymphoma. In both cases, immunohistochemistry (CD20+, CD5+, cyclin D1+, CD10-, and CD23-), and demonstration of clonal immunoglobulin heavy chain and bcl-1 gene rearrangements by PCR analysis confirmed the diagnosis of mantle cell lymphoma.